1. Field of the Invention
This invention relates in general to the field of wireless communications, and more particularly to a system and method of enhanced pilots for improved mobile position estimation in a wireless network.
2. Description of the Related Art
Mobile location or positioning services in a wireless communication system are useful for a variety of applications including emergency, commercial and personal applications. A mobile station may use any one of a variety of methods for determining its position. Although a Global Positioning System (GPS) using measurements from orbiting satellites may be a more accurate method of determining location, GPS may not be supported by the mobile station or may otherwise be non-functional in a local area in which line of sight to at least one satellite is not available.
Advanced Forward Link Trilateration (AFLT) is a mobile positioning method developed for cellular phones using Code Division Multiple Access (CDMA) technology, such as according to the CDMA200 family of third generation (3G) mobile technology standards, generally referred to herein as “1x”. According to the AFLT technique, the mobile station gathers cellular signals received from multiple base stations for measuring relative timing delays between the base stations. The collected measurements may then be sent to a nearby base station which calculates a location estimate. To determine location, the mobile station takes timing measurements of received cellular signals from nearby base stations (e.g., cellular towers) and reports the readings back to the network, which are then used to triangulate an approximate location of the mobile station. In general, at least three surrounding base stations may be used to obtain a position determination.
There are many signification factors which may affect AFLT positioning performance using 1x. 1x employs limited transmission bandwidth, such as 1.2288 mega-Hertz (MHz) per carrier (frequency). Cellular communications, including those based on the 1x standard, suffer from various noise sources or distortions, such as multipath propagation phenomenon (in which the same signal transmitted by a base station arrives at the mobile station at different times via multiple paths), co-channel interference and noise, dilution of precision (DOP) or geometric DOP (GDOP) (errors in measurement which affect overall solution), etc. The positioning estimation performance is quantified by “hearability,” which is a measure of the number of base stations that are “visible” to the mobile station (exacerbated by the near-far effect), and location accuracy or location error statistics.
A highly detectable pilot (HDP) structure, such as that described in US Patent Application Publication 2010/0074344 by Michael M. Wang et al., published Mar. 25, 2010, provided several benefits including improved hearability and improved positioning accuracy and reliability. The conventional HDP approach, however, added several challenges, including impact to existing or legacy 1x services, implementation cost increases, and positioning estimation delay.
Accordingly, it is desired to provide a method and apparatus that improves upon the conventional HDP approach at reduced cost and providing backwards compatibility with conventional or legacy systems.